The subject invention relates to digital printing. (As used herein, the term xe2x80x9cdigital printingxe2x80x9d refers to any form of printing wherein print control signals control a print mechanism to produce a matrix of pixels, i.e. picture elements, having two or more intensity values, to represent an image.) More particularly it relates to apparatus and methods for the real-time measurement of digital print quality.
Low cost, widely available digital printing technologies such as ink jet, bubble jet, and thermal transfer printing have enabled many new applications where dynamically varying information must be transmitted in printed form. Many of these applications rely upon a consistent level of print quality over time since the failure to capture the unique information on even a single document can have serious consequences.
A particular example of an application of digital printing where a consistent level of print quality is very important is the use of digital print mechanisms in postage meters and mailing machines (hereinafter sometimes postage metering systems). As is well known such devices print postal indicia on mailpieces as proof of the payment of postage. Upon payment to a proper authority such meters or machines are xe2x80x9cchargedxe2x80x9d with a representation of an equivalent amount of funds. As postal indicia are printed the funds in the meter are debited accordingly until exhausted. Since postal services accept indicia printed by postage meters or mailing machines as conclusive proof of payment of the amount of postage indicated such devices are in effect machines for printing money. As a result postal services have imposed high standards both on the print quality of indicia produced by such machines, and on the design of the machines themselves to assure that the appropriate amount is debited from the amount charged into the machine for each indicia printed.
Low cost digital print technologies have greatly simplified and improved the design of postage meters and mailing machines in many respects. Prior postage meters and mailing machines relied upon impact printing techniques which required complicated and expensive mechanisms to print varying postage amounts, which can now be printed in a simple, conventional manner with digital print mechanisms. More importantly, digital print mechanisms can be easily programmed to print other information such as security codes or addressing or tracking information with the postal indicia to facilitate automated mail handling. However such low cost digital print mechanisms can not easily provide consistent print quality as their mechanisms tend to degrade over time as ink dries up, small print nozzles clog or one or more of a number of small, rapidly cycling print elements fails. Such failure can cause substantial losses to a mailer since a large number of mail pieces of substandard print quality may be rejected by a postal service after the cost of the postage has been debited from the pre-paid amount charged to the machine.
U.S. Pat. No. 4,907,013; to: Hubbard et al.; issued: Mar. 6, 1990 is believed to be the prior art closest to the subject invention and relates to circuitry for detecting failure of one or more nozzles in an ink jet printhead. In Hubbard et al. a line containing one dot printed by each nozzle in the printhead is scanned to detect the possible absence of a dot. The line can form either a test pattern run before the start of a printing operation or can be incorporated into the image to be printed.
U.S. Pat. No. 5,038,208; to; Ichikawa et al.; issued: Aug. 6, 1991 teaches an ink jet printer which stores the image forming characteristics of an ink jet printhead and which corrects the image forming signals in accordance with the stored characteristics to maintain uniform print density.
U.S. Pat. No. 5,126,691; to: Millet et al.; issued: Jul. 7, 1992 is similar to Hubbard et al. in that it teaches a method for monitoring print quality by the use of a specially printed control frame.
U.S. Pat. No. 5,321,436; to: Herbert; issued Jun. 14, 1994 teaches a postage meter in which the operation of an ink jet printhead is checked by printing a predetermined bar code and then scanning the bar code to determine if it was correctly printed.
U.S. Pat. No. 5,473,351; to: Heterline et al. teaches a method and apparatus for monitoring print density by measuring printed line width and modifying the energy of the pulses applied to each ink jet nozzle to correct the line width.
While perhaps suitable for their intended purpose the print quality monitoring and control techniques found in the prior art did not provide a simple and inexpensive way to monitor print quality in real-time. Hubbard and similar prior art require special test patterns and so lack the immediate ability to detect a failure of print quality and/or the flexibility to monitor arbitrary print images; while other techniques taught in the prior art require expensive apparatus for measuring line width or printhead characteristics together with complicated control of the printhead drive signals.
Thus it is an object of the invention to provide an improved apparatus and method for the prompt, real-time monitoring of print quality so that prompt corrective actions can be taken.
The above object is achieved and the disadvantages of the prior art are overcome in accordance with the subject invention by means of a method and apparatus for monitoring print quality produced by a digital printing mechanism, wherein predetermined print control signals are provided to the digital printing mechanism, the printing mechanism responding to the print control signals to print an image on a substrate; a reference signal is derived from said print control signals; said image is scanned to generate a post-print signal; said reference signal is compared with said post-print signal; and if said reference signal and said post-print signal do not compare within predetermined standards, an output signal indicative of poor print quality is generated.
In accordance with one aspect of the subject invention the print mechanism is incorporated into a postage metering system and said image is a postal indicia.
In accordance with another aspect of the subject invention the postage meter is responsive to a signal generated as a function of the output signal to inhibit further printing of postal indicia.
In accordance with another aspect of the subject invention the comparison of the reference signal and the post-print signal is corrected to compensate for the reflectance of the substrate.
In accordance with another aspect of the subject invention, prior to printing the image, a substrate upon which the image is to be printed is scanned to generate a reflectance signal, the reflectance signal being used to correct the post-print signal.
In accordance with another aspect of the subject invention, a substrate upon which the image is to be printed has a uniform reflectance, and, prior to printing the segment, an overall reflectance signal for the substrate is determined, the overall reflectance signal being used to correct the post-print signal.
In accordance with another aspect of the subject invention, the post-print signal represents integration of reflectance over segments of the image. In accordance with another, related aspect of the subject invention, each of the segments comprises a plurality of pixels printed substantially concurrently by a printhead in the print mechanism.
In accordance with another aspect of the subject invention, the post-print signal represents summation of scanned pixels over segments of the image. In accordance with another, related aspect of the subject invention, each of the segments comprises a portion of the image printed by a separate, corresponding printhead comprised in the print mechanism. In accordance with another, related aspect of the subject invention each of the segments is scanned by a separate photosensor. In accordance with another, related aspect of the subject invention a plurality of the segments comprise a postal indicia.
It will be apparent to those skilled in the art that the above object is achieved and the disadvantages of the prior art are overcome by the subject invention. Other objects and advantages of the invention will be apparent from consideration of the detailed description set forth below and of the attached drawings.